Holographic system without laser speckle

ABSTRACT

A holographic system particularly for holographic interferometry which minimizes the effect of laser speckle yet permits viewing of the holographic image over a wide angular range at discrete angles. The scene beam passes a periodic grating structure, such as a wire mesh screen, a phase grating, or an amplitude grating for breaking up the scene beam into a number of discrete beams corresponding to different orders. The subject is interposed between the grating structure and the hologram. A lens may be arranged between the subject and the hologram for focusing the various discrete scene beams so that a desired number may be selected by an aperture plate. Alternatively, a pair of mirrors may be used for bringing together the discrete scene beams created by the grating structure. A further refinement consists of a second periodic grating structure disposed between the lens and the subject; again, the discrete scene beams may be focused by a lens. The two diffraction gratings preferably have different grating constants. Also, each of the discrete scene beams may be recorded on a separate hologram on which a reference beam is also directed. This may be effected by an additional periodic grating structure for also diffracting a discrete reference beam into the plane of each of the various holograms.

U uucu Dl'dl Brooks HOLOGRAPHIC SYSTEM WITHOUT LASERSPECKLE Gerritsen etal, 7 Applied Optics 230l- 2311 (1 H1968) Primary Examiner-DavidSchonberg Assistant Examiner-Robert L. Sherman Attorney-Daniel T.Anderson, Edwin A. Oser and Jerry Dinardo 51 May 2, 1972 57] ABSTRACT Aholographic system particularly for holographic interferometry whichminimizes the effect of laser speckle yet permits viewing of theholographic image over a wide angular range at discrete angles. Thescene beam passes a periodic grating structure, such as a wire meshscreen, a phase grating, or an amplitude grating for breaking up thescene beam into a number of discrete beams corresponding to differentorders. The subject is interposed between the grating structure and thehologram. A lens may be arranged between the subject and the hologramfor focusing the various discrete scene beams so that a desired numbermay be selected by an aperture plate. Alternatively, a pair of mirrorsmay be used for bringing together the discrete scene beams created bythe grating structure. A further refinement consists of a secondperiodic grating structure disposed between the lens and the subject;again, the discrete scene beams may be focused by a lens. The twodiffraction gratings preferably have different grating constants. Also,each of the discrete scene beams may be recorded on a separate hologramon which a reference beam is also directed. This may be effected by anadditional periodic grating structure for also diflracting a discretereference beam into the plane of each of the various holograms.

20 China, 7 Drawing Figures Patented May 2, 1972 3,659,914

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Relative lnfenslfy of the Compound Orders for Two Sinusoidal 6 PhaseGratings I I 7.0 -6LO-50-4 .O-3.0-20 -I.O 0.0 L0 20 3.0 4.0 5.0 6.0 20--Compound Order Number- Robert E Brooks INVENTOR.

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ATTORNEY Patented May 2, 1972 3 Sheets-Sheet 2 FIG. 7

Robert E. Brooks m w. mlmv 0 6 Y B ATTORN EY Patented May 2, 1972 3Sheets-Sheet 5 Robert E. Brooks INVENTOR. BY i 's 0.

ATTORNEY l HOLOGRAPHIC SYSTEM WITHOUT LASER SPECKLE BACKGROUND OF THEINVENTION This invention relates generally to holographic systems, andparticularly relates to a system suitable particularly for holographicinterferometry which minimizes the effects of laser speckle and pennitsviewing of the holographic image over a wide angular field at discreteangles.

One of the outstanding and dramatic features of holography is that thesubject recorded can be reconstructed over a wide angular range from thesame hologram. This is specially advantageous when such a wide anglehologram is used for holographic interferometry. lt makes it possible toobtain a number of different interferograms, each being a reconstructionof the original hologram and each corresponding to a different viewingangle. Such wide-angle viewing has been obtained in the past byilluminating the subject with diffused light; that is, by means of adiffusing screen or by using a subject which diffusely reflects thelight.

However, the use of diffused light in holography has a major drawback:the random phase of the subject light or scene beam at the plane of thehologram gives rise to an image which is modulated by a granularamplitude pattern, usually called laser speckle, laser granularity, orsometimes worms. The size of the granularity is inversely proportionalto the size of the imaging aperture such as a lens or the pupil of theviewer. On the other hand, to obtain depth of field, one usually has touse a small imaging aperture. Accordingly, it will be obvious thatfreedom from granularity and depth of field of the imaging system ormutually incompatible. This becomes particularly important where theinterference fringes of the holographic interferometric reconstructionare located in space, and where it is desired to image sharply on thefringe pattern and on the subject.

The drawbacks of laser speckle have been known for some time and variousproposals have been made in the past for eliminating the speckle noise.To this end, the use of a grating has been proposed in a paper byGerritsen, Hsnnan, and Ramberg which appears in the November 1968 issueof Applied Optics, Volume 7, No. ll, pages 2,30l-2,3l 1. In this paper,it is proposed to provide a holographic system consisting of a grating,a lens, and a transparency so that the diffraction grating is in contactwith the lens-transparency combination. The purpose of this arrangementis to increase the amount of redundancy in recording holograms; that is,a plurality of holograms are recorded at the same time and on the samerecording medium. The diffraction grating breaks up the light beam intoa plurality of discrete beams, each of which is recorded on the samehologram. The lens is not used for separating the discrete scene beams.While the hologram is not at the focal plane of the lens, it is veryclose.

Various types of gratings have been proposed, such as ab sorptiongratings, phase gratings, and ordinary or diffraction gratings. Also,two dimensional gratings have been proposed where the grating linesintersect each other at right angles for the purpose of increasing theredundancy of the hologram by providing a two-dimensional set ofholograms. It should also be noted that the object in this case is atwo-dimensional transparency rather than a three-dimensional realobject. Thus, it would not be possible to view the two-dimensionalobject of the Gerritsen et al. paper at different angles to obtain athreedimensional aspect of the object.

The hologram obtained in accordance with the Gerritsen et al. paper isreproduced so that all of the separate images are reproduced together.The purpose, of course, is to increase the redundancy and to make thehologram more immune to mechanical damage such as scratches.

Another system for eliminating laser speckle in holographicinterferograms has been proposed in a paper by Vest and Sweeney whichappears in the October 1970 issue of Applied Optics, Volume 9, No. 10,pages 2,32 l-2,325. This paper also proposes the use of a phase grating.However, in this case, the discrete subject beams which are created bythe grating are not focused near the hologram. This requires aratherlarge angle between the reference beam and the discrete scenebeams simply due to the geometric arrangement. ln accordance with thepresent invention, a relatively small angle between the reference andscene beams may be obtained, as disclosed in the applicants prior U. S.Pat. No. 3,533,675, which is assigned to the assignee of the presentinvention.

The Vest et al. paper also proposes to reconstruct the hologram byviewing it through a transforming lens and a filtering aperture. Thiswould tend to screen out all but a particular discrete viewing anglewhich has originally been recorded on the hologram.

lt is accordingly an object of the present invention to provide aholographic system particularly suitable for holographic interferometrywhich minimizes or substantially eliminates the effects of laser speckleand still permits viewing of the holographic image over a wide angularrange at discrete angles.

Another object of the invention is to provide a holographic system ofthe type referred to where the grating constants of a pair of gratingsmay be selected in such a manner that the energies of the variouscompound orders are equal within an order of magnitude over a desiredangular range and are spaced so as to minimize overlap of adjacentorders.

A further object of the present invention is to provide a holographicinterferometric system of the type referred to which permits to record anumber of separate holograms, one for each discrete subject beam.

SUMMARY OF THE INVENTION A holographic system for performinginterferometry in accordance with the present invention minimizes orsubstantially eliminates the effects of laser speckle. It also permitsviewing of the holographic image over a wide angular range at discreteangles. The system comprises a laser for generating a laser beam andmeans for dividing the laser beam into a reference and a scene beam.Means are also provided for recombining the beams in a predetenninedplane. A recording medium, such as a photographic film or plate, or aphotochromic material, is provided in the predetermined plane.

Further in accordance with the present invention, a periodic gratingstructure is disposed in the path of the scene beam and ahead of anon-diffuse subject to be holographed. The grating structure breaks upthe scene beam into a number of discrete beams corresponding todifferent orders. Such a periodic grating structure may consist, forexample, of a wire mesh screen, a diffraction grating having ruledparallel lines, a phase grating, or an amplitude grating. Finally, afocusing lens may be disposed in the path of the discrete scene beamsbetween the subject and the recording medium for focusing the variousdiscrete scene beams created by the grating structure. An aperture platemay be disposed between the lens and the recording medium andsubstantially in the focal plane of the lens. The aperture of the plateis arranged for passing only predetermined orders corresponding to thediscrete scene beams. Thus, the scene beam is not diffused.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims.

The invention itself, however, both as to its organization and method ofoperation, as well as additional objects and advantages thereof, willbest be understood from the following description when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of aholographic system embodying the present invention and including aperiodic grating structure for breaking up the subject beam and afocusing lens for subsequently focusing the discrete subject beams;

FIG. 2 is a schematic view of a system for reproducing a previotnlyrecorded hologram and for selecting a desired one of the recordeddiscrete subject beams;

FIG. 3 is a schematic view of another system for reproducing orreconstructing a recorded hologram by means of a camera;

FIG. 4 is a schematic view of a modified embodiment of the presentinvention including a pair of mirrors for reflecting the scene beams,thereby to bring the focal points of the discrete subject beam within asmall area;

H6. 5 is a schematic showing of a preferred embodiment of the inventionfeaturing two separate, periodic grating structures;

P10. 6 is a graph showing the relative intensities of various compoundorders obtainable with a particular set of two diffraction gratings ofthe system of HO. 5; and

FIG. 7 is a schematic view of still another embodiment of the inventionwhere separate recording media are provided for recording separate scenebeams obtained by a first diffraction grating, and where a seconddiffraction grating is used for also breaking up the reference beam intoindividual beams.

DESCRIPTlON OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and particularly to FIG. 1, there is illustrated a holographic systemembodying the present invention. The system includes a laser 10 forgenerating a monochromatic and coherent light beam 11. The laser beam 11may then be enlarged by an enlarging or negative lens 12. The resultingenlarged beam is then collimated by a collimating or positive lens 14,the lenses 12 and 14 forming an inverted Galilean telescope.

The resulting laser beam 15 may now be split or divided into a referencebeam 16 and a scene beam 17. As shown in FIG. 1, this may be effectedsimply by utilizing a portion of the collimated beam 15 as a referencebeam 16. The reference beam 16 may now be bent by a prism 18, and may befocused by a focusing lens 20 in a focal point 21 located in the planeof an aperture plate 22 having an aperture 23. The reference beam 16,past its focal point 21, enlarges again to illuminate the plane formedby a recording medium 24 on which the hologram is to be recorded. Therecording medium 24 may consist of a photographic film or plate, aphotochromic material, or any other suitable light recording substance.

The scene beam 17 passes through a periodic grating structure 25. Thisstructure 25 may consist, for example, of a wire mesh screen, aconventional diffraction screen having ruled parallel lines, a phasegrating, or an amplitude grating. Periodic grating structures are wellknown in the art. A phase grating has the property of changing theeffective optical path length of the light passing therethrough in aperiodic fashion. This may be effected, for example, by a periodicchange of the thickness of the grating or by a change of the refractiveindex thereof. An amplitude grating is sometimes called an absorptiongrating and varies periodically the amplitude or intensity of the lightwave passing therethrough without changing the phase of the wave. Suchan absorption grating may have a twodimensional, sinusoidally varyingamplitude transmission. Diffraction gratings, of course, have long beenknown and consist of a transparent substance, such as glass, or of areflecting substance, such as metal, on which a multiplicity of parallellines are ruled.

The diffracting screen 25 breaks up the scene beam 17 into a pluralityof individual and discrete scene beams such as 26, 27 and 28. The scenebeam now passes through a subject 30 to be holographed, and thereafter,through a focusing lens 32. Each of the discrete scene beams 26, 27 and28 is now focused by the lens 32 in the plane of the aperture plate 21at focal points 34, 35 and 36. These separate images of the subject 30are then recorded together on the hologram 24. It will be noted that thephysical arrangement is such that a small angle between the referencebeam 16 and the discrete scene beams 26 28 may be maintained. This isdesirable because it permits the use of photographic emulsion with lowresolution but high light sensitivity. The aperture 23 in the apertureplate 22 is of such a size as to eliminate unwanted scene beams, such asthe scene beam 38. Thus, depending on the size of the aperture 23, moreor fewer of the discrete scene beams may be recorded on the hologram 24.

It should be noted that the holographic system of the present inventionis particularly suitable for performing holographic interferometry. Forexample, changes in a subject such as 30 which may occur due to appliedstress or strain, or due to the movement of expanding gases and thelike, may be recorded at two subsequent instants of time on the samehologram. This is the so-called double-exposure holographic technique.Alternatively, the changes of the subject 30 may be viewed by looking atthe subject through the hologram 24 whereby interference fringes appearto indicate any changes between the original subject and the subject ata later time. Alternatively, a periodically changing subject may beholographed over a relatively extended period of time to obtain the timeaverage of the motion or vibration whereby nodal and ventral points ofthe vibration stand out in the reconstructed hologram.

As indicated before, the reference beam preferably is arranged in such amanner that it appears as a point source 21 in the plane of the apertureplate 22. This, of course, makes it possible to locate the referencebeam source 21 physically close to the focal points 34 36 of thediscrete scene beams.

The use of the focusing lens 32 has the advantage that the discretescene beams are brought together again so that their focal points 34 36are closely adjacent to each other. Furthermore, the lens 32 directs thelight toward the hologram 24 so that less of the light is wasted.

Preferably, the aperture plate 22 is at or near the focal plane of thelens 32. The hologram 24 is preferably located behind the focal plane ofthe lens 32 but is not in the focal plane of the lens.

It will now be appreciated that a plurality of separate images isrecorded on the hologram 24 and that each separate image initiallyrecorded by one of the discrete scene beams may now be separatelyreconstructed and viewed. This may be effected, for example, by theapparatus of F IG. 2. This shows again the recording medium 24' which isnow a developed hologram. it is illuminated again with the referencebeam 16 through a focusing lens 20 so that a point reference source 21is created which illuminates the hologram 24. The conjugate image of thereconstructed hologram may be viewed through an aperture plate 40 by anobserver, indicated by the eye 41. The aperture 42 of the aperture plate40 is such as to select one of the discrete scene beams corresponding,for example, to the focal point 34. Preferably, the image reconstructedfrom the hologram 24 is focused by a lens 44.

Thus, it will be seen that a plurality of separate holographic images isrecorded on the original hologram 24, each of which may now beseparately viewed through the aperture plate 40. This system ensuresthat the holographic image may be viewed over a wide angular range butat discrete angles. On the other hand, the effects of laser speckle havebeen minimized or substantially eliminated.

The hologram 24 may also be reproduced as shown in H6. 3 by a referencebeam 16, which is the conjugate of the reference beam, by means of whichthe hologram was recorded. In this case, the real image is reconstructedat the focal points 34, 35 and 36. By means of the aperture 42 in theaperture plate 40, one of the discrete scene beams may be selected andmay now be photographed by a photographic camera 46.

In some cases, the discrete scene beams created by the diffractingscreen 25 may have widely separated angles. This, of course, is due tothe grating constant of the diffracting screen, where the gratingconstant is defined as the distance between adjacent lines or otherdiscontinuities which constitute the periodic grating structure. In thatcase, it may be desirable to redirect the focal points of the respectivediscrete scene beams so that they fall together in a plane and closelyadjacent to each other. This avoids the necessity of using a large-areaphotographic plate 24. An apparatus for accomplishing this has beenshown in FIG. 4. Here, the original laser beam passes through thediffracting screen 25, then onto the subject 30 and the focusing lens32. The various discrete scene beams may now be focused by the lens 32at the points 50. 51 and 52. It will be seen from F IG. 4 that they arewidely separated and that a large-area hologram would be required torecord them all. On the other hand, it is usually desired to have arelatively small-area hologram 24.

Accordingly, in accordance with the present invention, a pair of minors53 and 54 is provided for folding the focal points 50 and 52 byreflection so that they now fall at the points 55 and 56; that is,closely adjacent to each other. For example, FIG. 4 shows that the rays57 and 58, which are focused at the point 50, are reflected by themirror 53 to become, respectively, rays 60 and 61 which now intersect atthe focal point 55. The same is true of the rays which form the focalpoint 52.

Accordingly, the focused, discrete scene beams may now be recorded onthe hologram 24. A reference beam may have an origin or point source 21so as to form a small angle between the reference and scene beams at thehologram, thereby reducing the requirement for a high resolutionrecording medium. For the sake of simplicity, in FIG. 4, neither thelaser source nor the means for creating a reference point source 21 havebeen shown. However, it will be understood that this may be effected byany conventional means; for example, a lens and a pair of mirrors may beused.

It will also be understood that an aperture plate may again be providedin the plane defined by the focal points 51, 55 and 56 to eliminateadditional unwanted discrete scene beams.

A preferred embodiment of the present invention is illustrated in H0. 5,to which reference is now made. Instead of providing only a singleperiodic grating structure or diffraction grid, there are now providedtwo such structures or diffraction grids. This permits greaterflexibility of the system and permits to control, within certain limits,the angle or separation of the discrete beams and their relativeintensities. lt is also feasible to control the degeneracy or overlap ofthe compound orders of the discrete output scene beams.

As shown in FIG. 5, the laser input beam 15 passes through a firstperiodic grating structure 25, then the subject 30, and a secondperiodic grating structure 65. As will be subsequently shown, thiscreates two sets of discrete scene beams which now impinge on thefocusing lens 32 which focuses the respective scene beams in a focalplane in or near which the aperture plate 22 is provided so that theaperture 23 selects the desired orders or particular discrete scenebeams which it is desired to record. Additionally, the point 21 againindicates the point source of the reference beam which also illuminatesthe hologram 24.

Thus, the collimated laser beam I5 is split up by the first grating 25into a plurality of discrete beams such as 66, 67 and 68. Each of thesethree beams 66, 67 and 68 is again split up or broken up into aplurality of discrete scene beams such as 70, 71 and 72, which, by wayof example, are created by the first discrete scene beam 66. One of thesecond set of discrete scene beams, such as 71, will now be focused bythe focal lens 32 in a focal point 73. The same, of course, is true ofother sets of discrete beams. Furthermore, there will be one set offocal points, such as 73, within the aperture 23, and a second set offocal points, such as 74, which is disposed a distance from the firstset of focal points and therefore is intercepted by the aperture plate22.

The two periodic grating structures 25 and 65 may again consist of anyof the various diffraction gratings previously discussed. lt willsubsequently be shown that they preferably have different gratingconstants. The direction of the lines or of the periodicities of the twogratings may be rotated with respect to each other, or else one of thegratings may be tilted with respect to the other.

ln general, it might be said that the ratio of the wavelength of thelight to the grating constant determines the angles between adjacentbeams. For small angles, the angular separation is approximatelyconstant. It can also be shown that, for two diffraction gratings whichare assumed to be parallel to each other, the diffraction angles aredetermined by the ratio of the wavelength to the grating constant.multiplied by an effective or compound order number which depends on theratio of the two grating constants.

lt can also be shown that the energy in the various diffracted orders orcompound orders is a function of A X B, where A and B are the amplitudesof the phase excursions of the two gratings which vary in apredetermined direction.

Thus, FIG. 6 shows, by way of example, the intensity and relativespacings of the compound order numbers of such a double grating. Forthis example, the values of A and B have each been chosen to be equal tothree, while the ratio of the two grating constants has been selected tobe H. Thus, the energy in each of the diffracted orders depends on thevalues of A and B and may be selected for any particular purpose.Rotating one of the gratings with respect to another facilitatesseparation of the respective orders. lf the gratings are made such thatthey are not simple sinusoidal gratings but have, for example,non-linear exposure versus phase relationship, higher spatial frequencyharmonics will result. This, of course, will change the entiredistribution of energy among the orders. Accordingly, it will be seenthat, by suitably selecting the properties of the two gratings 25 and65, different desirable results may be obtained.

Still another embodiment of the invention is illustrated in FIG. 7, towhich reference is now made. Here, a separate hologram is recorded ofeach of the individual or discrete scene beams. Thus, the light from alaser 10 falls on a conventional beam splitter to provide a scene beam81 and a reference beam 82. The scene beam 81 then falls on an object 30and subsequently on a diffraction grating 83 which splits up the scenebeam into say three discrete scene beams which may now be recorded bythree recording media 84, 85 and 86. The reference beam 82 is reflectedby the mirror 87 and falls on a second diffraction grating 88 whichsimilarly splits up the reference beam into three separate referencebeams so that a scene beam and a reference beam impinge on each of thethree holograms 84, 85 and 86. In this case, each of the discrete scenebeams is recorded on its own hologram, and may be reconstructedtherefrom. Since there are separate holograms for each discrete scenebeam, there must also be a separate reference beam for each hologram. lnF IG. 7, this has been accomplished by the grating 88. Alternatively,the same results may be obtained by using a plurality of successive beamsplitters and mirrors to split up the reference beam into a plurality ofseparate beams, each of which is directed toward the appropriatehologram.

There has thus been disclosed a holographic system particularly suitablefor holographic interferometry. it permits to substantially eliminatethe effects of laser speckle while still maintaining thethree-dimensional aspect of the reconstructed image. This is effected bythe use of a single diffraction grating with a focusing lens or else bythe use of two diffraction gratings. Alternatively, each of the discretescene beams which are created by the use of a difl'raction grating maybe separately recorded on its own hologram. The use of two diffractiongratings lends great flexibility to the system and permits, withinlimits, to control both the overlap or spatial separation of the orders,as well as their energy distribution.

What is claimed is:

l. A holographic system for perfonning interferometry and for minimizingthe effects of laser speckle and permitting viewing of a holographicimage of a three-dimensional subject over a wide angular range atdiscrete angles, said system compnsmg:

a. a laser for generating a laser beam;

b. means for dividing said laser beam into a reference beam and a scenebeam and for recombining said beams in a predetermined plane;

c. a recording medium disposed in said predetermined plane for recordinga hologram;

d. a periodic grating structure disposed in the path of said scene beamand ahead of a three-dimensional subject to be holographed for breakingup said scene beam into a number of discrete beams corresponding todifferent orders; and

e. a focusing lens disposed in the path of said discrete scene beamsbetween the subject and said recording medium for focusing the variousdiscrete scene beams created by said grating structure.

2. A holographic system as defined in claim 1 wherein an aperture plateis disposed between said lens and said recording medium andsubstantially in the focal plane of said lens for passing onlypredetermined orders of said discrete scene beams.

3. A holographic system as defined in claim 1 wherein a pair of mirrorsis provided substantially between said focusing lens and said recordingmedium, said minors being disposed for reflecting on said recordingmedium desired discrete scene beams created by said grating structure.

4. A holographic system as defined in claim 1 wherein a second periodicgrating structure is disposed in the path of said scene beam between thesubject and said lens, said second grating structure providing anadditional set of discrete beams which are recombined with thefirst-named discrete beams at said recording medium.

5. A holographic system as defined in claim 1 wherein said periodicgrating structure consists of a wire mesh screen.

6. A holographic system as defined in claim 1 wherein said gratingstructure consists of a support having ruled parallel lines.

7. A holographic system as defined in claim 1 wherein said gratingstructure is a phase grating.

8. A holographic system as defined in claim 1 wherein said gratingstructure is an amplitude grating.

9. A system for performing holographic interferometry and for minimizingthe effects of laser speckle, said system permitting viewing of aholographic image of a three-dimensional subject over a wide angularfield at discrete angles and comprising:

a. a laser for generating a laser beam;

b. means for dividing said laser beam into a reference beam and a scenebeam and for recombining said beams in a predetermined plane;

. a recording medium disposed in said predetermined plane for recordinga hologram;

cl. a first periodic grating structure disposed in the path of saidscene beam and ahead of a three-dimensional subject to be holographed;and

. a second periodic grating structure disposed in the path of said scenebeam and between the subject and said recording medium, each of saidgrating structures having a different grating constant and breaking upsaid scene beam into a number of discrete beams corresponding todifferent orders, said discrete beams being recombined at said recordingmedium.

10. A holographic system as defined in claim 9 wherein a focusing lensis disposed in the path of said discrete scene beams between said secondgrating structure and said recording medium for focusing the variousdiscrete scene beams created by said grating structures.

ll. A holographic system as defined in claim 10 wherein an apertureplate is disposed between said lens and said recording medium andsubstantially in the focal plane of said lens for passing onlypredetermined. orders of said discrete scene beams.

12. A holographic system as defined in claim 9 wherein each of saidgrating structures consists of a wire mesh screen.

13. A holographic system as defined in claim 9 wherein each of saidgrating structures consists of a diffracting grating having niledparallel lines.

14. A holographic system as defined in claim 9 wherein the periodicstructure of one of said gratings is rotated with res ect to that of theother gratin 5. A holographic system as defined in claim 9 wherein oneof said grating structures is tilted with respect to the other.

16. A holographic system as defined in claim 9 wherein each of saidgrating structures is a phase grating.

17. A holographic system as defined in claim 9 wherein each of saidgrating structures is an amplitude grating.

18. A holographic system for minimizing the eflects of laser speckle andfor permitting viewing of the holographic image of a three-dimensionalsubject at different discrete, widely separated angles, said systemcomprising:

a. a laser for generating a laser b'ea b. means for splitting said laserbeam into a reference beam and a scene beam;

c. a first periodic grating structure disposed in the path of said scenebeam and ahead of a three-dimensional subject to be holographed forbreaking up said scene beam into a number of discrete scene beams;

d. means for directing said reference beam into the path of each of saiddiscrete scene beams; and

e. a plurality of recording media, each being disposed in apredetermined plane and in the path of one of said discrete scene beamsand one of said reference beams. whereby each of said recording mediamay be reconstructed to view the subject at a different discrete angle.

19. A holographic system as defined in claim 18 wherein said means fordirecting said reference beam consists of an additional periodic gratingstructure disposed in the path of said reference beam and for breakingup said reference beam into a number of discrete reference beams, eachbeing directed into the path of one of said discrete scene beams.

20. A holographic system for perfonning interferometry and forminimizing the efi'ects of laser speckle and permitting viewing of aholographic image of a three-dimensional subject over a wide angularrange at discrete angles, said system comprising:

a. a laser for generating a laser beam;

b. means for dividing said laser beam into a reference beam and a scenebeam and for recombining said beams in a predetermined plane;

c. a recording medium disposed in said predetermined plane for recordinga hologram;

d. a periodic grating structure disposed in the path of said scene beamand ahead of a three-dimensional subject to be holog'raphed for breakingup said scene beam into a number of discrete beams corresponding todifferent orders;

e. means for developing the exposed recording medium to provide ahologram and for repositioning it; and

f. means for illuminating said hologram with a reference beam and forpermitting viewing of one of the reconstructed discrete scene beams.

i 9 Q i

1. A holographic system for performing interferometry and for minimizingthe effects of laser speckle and permitting viewing of a holographicimage of a three-dimensional subject over a wide angular range atdiscrete angles, said system comprising: a. a laser for generating alaser beam; b. means for dividing said laser beam into a reference beamand a scene beam and for recombining said beams in a predeterminedplane; c. a recording medium disposed in said predetermined plane forrecording a hologram; d. a periodic grating structure disposed in thepath of said scene beam and ahead of a three-dimensional subject to beholographed for breaking up said scene beam into a number of discretebeams corresponding to different orders; and e. a focusing lens disposedin the path of said discrete scene beams between the subject and saidrecording medium for focusing the various discrete scene beams createdby said grating structure.
 2. A holographic system as defined in claim 1wherein an aperture plate is disposed between said lens and saidrecording medium and substantially in the focal plane of said lens forpassing only predetermined orders of said discrete scene beams.
 3. Aholographic system as defined in claim 1 wherein a pair of mirrors isprovided substantially between said focusing lens and said recordingmedium, said mirrors being disposed for reflecting on said recordingmedium desired discrete scene beams created by said grating structure.4. A holographic system as defined in claim 1 wherein a second periodicgrating structure is disposed in the path of said scene beam between thesubject and said lens, said second grating strUcture providing anadditional set of discrete beams which are recombined with thefirst-named discrete beams at said recording medium.
 5. A holographicsystem as defined in claim 1 wherein said periodic grating structureconsists of a wire mesh screen.
 6. A holographic system as defined inclaim 1 wherein said grating structure consists of a support havingruled parallel lines.
 7. A holographic system as defined in claim 1wherein said grating structure is a phase grating.
 8. A holographicsystem as defined in claim 1 wherein said grating structure is anamplitude grating.
 9. A system for performing holographic interferometryand for minimizing the effects of laser speckle, said system permittingviewing of a holographic image of a three-dimensional subject over awide angular field at discrete angles and comprising: a. a laser forgenerating a laser beam; b. means for dividing said laser beam into areference beam and a scene beam and for recombining said beams in apredetermined plane; c. a recording medium disposed in saidpredetermined plane for recording a hologram; d. a first periodicgrating structure disposed in the path of said scene beam and ahead of athree-dimensional subject to be holographed; and e. a second periodicgrating structure disposed in the path of said scene beam and betweenthe subject and said recording medium, each of said grating structureshaving a different grating constant and breaking up said scene beam intoa number of discrete beams corresponding to different orders, saiddiscrete beams being recombined at said recording medium.
 10. Aholographic system as defined in claim 9 wherein a focusing lens isdisposed in the path of said discrete scene beams between said secondgrating structure and said recording medium for focusing the variousdiscrete scene beams created by said grating structures.
 11. Aholographic system as defined in claim 10 wherein an aperture plate isdisposed between said lens and said recording medium and substantiallyin the focal plane of said lens for passing only predetermined orders ofsaid discrete scene beams.
 12. A holographic system as defined in claim9 wherein each of said grating structures consists of a wire meshscreen.
 13. A holographic system as defined in claim 9 wherein each ofsaid grating structures consists of a diffracting grating having ruledparallel lines.
 14. A holographic system as defined in claim 9 whereinthe periodic structure of one of said gratings is rotated with respectto that of the other grating.
 15. A holographic system as defined inclaim 9 wherein one of said grating structures is tilted with respect tothe other.
 16. A holographic system as defined in claim 9 wherein eachof said grating structures is a phase grating.
 17. A holographic systemas defined in claim 9 wherein each of said grating structures is anamplitude grating.
 18. A holographic system for minimizing the effectsof laser speckle and for permitting viewing of the holographic image ofa three-dimensional subject at different discrete, widely separatedangles, said system comprising: a. a laser for generating a laser beam;b. means for splitting said laser beam into a reference beam and a scenebeam; c. a first periodic grating structure disposed in the path of saidscene beam and ahead of a three-dimensional subject to be holographedfor breaking up said scene beam into a number of discrete scene beams;d. means for directing said reference beam into the path of each of saiddiscrete scene beams; and e. a plurality of recording media, each beingdisposed in a predetermined plane and in the path of one of saiddiscrete scene beams and one of said reference beams, whereby each ofsaid recording media may be reconstructed to view the subject at adifferent discrete angle.
 19. A holographic system as defined in claim18 wherein said means for directing said reference beam consists of anadditional periodic grating strUcture disposed in the path of saidreference beam and for breaking up said reference beam into a number ofdiscrete reference beams, each being directed into the path of one ofsaid discrete scene beams.
 20. A holographic system for performinginterferometry and for minimizing the effects of laser speckle andpermitting viewing of a holographic image of a three-dimensional subjectover a wide angular range at discrete angles, said system comprising: a.a laser for generating a laser beam; b. means for dividing said laserbeam into a reference beam and a scene beam and for recombining saidbeams in a predetermined plane; c. a recording medium disposed in saidpredetermined plane for recording a hologram; d. a periodic gratingstructure disposed in the path of said scene beam and ahead of athree-dimensional subject to be holographed for breaking up said scenebeam into a number of discrete beams corresponding to different orders;e. means for developing the exposed recording medium to provide ahologram and for repositioning it; and f. means for illuminating saidhologram with a reference beam and for permitting viewing of one of thereconstructed discrete scene beams.